SYSTEM APPROACH ON DESIGNING AN OFFSHORE WINDPOWER GRID CONNECTION K. Eriksson, ABB Utilities Ludvika, Sweden; P. Halvarsson, ABB Utilities Västerås, Sweden; D. Wensky, ABB Utilities, Mannheim; Germany, M. Häusler, Weinheim, Germany Introduction The deregulation of the electricity market has led to new challenges. In a competitive environment the rules for investment are influenced strongly by new types of generating facilities such as gas fired plants, wind farms and other renewables. Up to now almost all wind farms are located onshore. In Germany for example at the end of 2002 there were installed some 13500 windpower turbines with an installed power of more than 12 000 MW Because of the power values and trying to minimize investments this new type of generation normally enters the grid at sub-transmission levels. (≤ 110 kV). Now sites suitable for addition al wind farms onshore have b ecome scarce in many Europe an countries.. Therefore the major efforts today are directed to develop future wind farms offshore. In Denmark, UK and a few other countries the first of fshore windfarms are already installed. Because of the size of the coming offshore wind farms it will be necessary to connect them to the high voltage level of the transmission system (in Europe normally at the 400 kV level). A considerable part of planned offshore wind farms will be located far away from shore. Connecting them to the grid at the nearest possible point normally adds some more kilometers onshore to the distance of the wind farm to shore. Taking into account the main parameters power and transmission distance, the connection for an offshore wind farm needs to be optimized case by case. These power transmission systems are normally comprised of a lot of submarine and land cables, AC switchgear, HVDC substations or AC compensation equipment, power transformers, OH lines, auxiliary equipment. Sometimes discussion is ongoing regarding the application of VSC based t echnology or ofconventional HVDC for the converters. For the latter type of converter also synchronous condensers may be needed offshore. The total cost of the different solutions vary considerably depending on the chosen configuration. To reach the optimum solution of the power transmission a true s ystem approach is necessary. This system approach requires a broad and at the same time detailed knowledge of the available main components and the capability to choose among them for the best suited devices with respect to the optimum overall function. So manufacturers in charge of a complete transmission system have a decisive advantage compared with those whose facilities cover only some individual components. This advantage becomes even more important, if the connections of several neighboring wind farms to the onshore grid are to be optimized. Since repair of offshore equipment is long lasting and expensive and reduces the availability of the wind farm, it is important to design for adequate reliability. Thus the following aspects should be considered : -use of components with a low failure rate -redundancy to continue operation even at single component level failure -provision for a short meantime to repair.
SYSTEM APPROACH ON DESIGNING AN OFFSHORE WINDPOWERGRID CONNECTION
K. Eriksson, ABB Utilities Ludvika, Sweden; P. Halvarsson, ABB Utilities Västerås, Sweden;D. Wensky, ABB Utilities, Mannheim; Germany, M. Häusler, Weinheim, Germany
IntroductionThe deregulation of the electricity market has led to new challenges. In a competitiveenvironment the rules for investment are influenced strongly by new types of generatingfacilities such as gas fired plants, wind farms and other renewables. Up to now almost allwind farms are located onshore. In Germany for example at the end of 2002 there wereinstalled some 13500 windpower turbines with an installed power of more than 12 000 MWBecause of the power values and trying to minimize investments this new type of generationnormally enters the grid at sub-transmission levels. (≤ 110 kV).
Now sites suitable for additional wind farms onshore have become scarce in many Europeancountries.. Therefore the major efforts today are directed to develop future wind farmsoffshore. In Denmark, UK and a few other countries the first offshore windfarms are alreadyinstalled. Because of the size of the coming offshore wind farms it will be necessary toconnect them to the high voltage level of the transmission system (in Europe normally at the400 kV level).A considerable part of planned offshore wind farms will be located far away from shore.Connecting them to the grid at the nearest possible point normally adds some more kilometersonshore to the distance of the wind farm to shore. Taking into account the main parameterspower and transmission distance, the connection for an offshore wind farm needs to be
optimized case by case.These power transmission systems are normally comprised of a lot of submarine and landcables, AC switchgear, HVDC substations or AC compensation equipment, powertransformers, OH lines, auxiliary equipment.Sometimes discussion is ongoing regarding the application of VSC based technology or of conventional HVDC for the converters. For the latter type of converter also synchronouscondensers may be needed offshore.The total cost of the different solutions vary considerably depending on the chosenconfiguration. To reach the optimum solution of the power transmission a true systemapproach is necessary. This system approach requires a broad and at the same time detailedknowledge of the available main components and the capability to choose among them for thebest suited devices with respect to the optimum overall function. So manufacturers in chargeof a complete transmission system have a decisive advantage compared with those whosefacilities cover only some individual components. This advantage becomes even moreimportant, if the connections of several neighboring wind farms to the onshore grid are to beoptimized.Since repair of offshore equipment is long lasting and expensive and reduces the availabilityof the wind farm, it is important to design for adequate reliability. Thus the following aspectsshould be considered :- use of components with a low failure rate- redundancy to continue operation even at single component level failure- provision for a short meantime to repair.
If a component is heavy or needs, after a major fault, to be shipped for repair to the factory,sufficient facilities and spare parts must be provided onboard the platform. This aspectdetermines to a great extent the design of the offshore station and platform. Therefore, asystem approach in the early planning stage is needed, to provide acceptable availability andto avoid high costs at a later stage.
Support for the electrical power transmission gridIn order to understand the challenge of large offshore wind farms connected to the electricalpower grid one has to be aware of how the grids are organized today and how they willprobably be organized in the future.
Traditionally, the power transmission grid is designed as a vertically integrated system. Theelectrical power, at the place of generation transformed to high voltages, is transmitted overhigh voltage lines to the distribution systems and then to the loads. The European UCPTEpower system is organized and operated country wise. Each country is responsible for thebalance of its own production and consumption. In comparison to the total installed powerrelatively weak connections exist between the different national grids. These connections aremainly used for support in case of emergency. So the mean distances between generation andload are below or in the range of 100 km. The existing network in Europe is not designed totransmit bulk power over long distances. In the future with the infeed of some 10 000 MWinstalled offshore wind power transmission distances may probably reach 1000 km or more.This means that a new 400 kV infrastructure needs to be developed. Since HVDCtransmission lines need less right-of-way than AC lines it is also possible that an overlayingHVDC grid will be developed and built as the most economic solution..
When the power arrives at a substation the voltage is transformed to a lower level and a sub-transmission system carries the power further to a distribution substation where the voltageagain is transformed and electrical power is distributed to the consumers. This vertical systemis well managed both with respect to investments and in the way it is controlled. Voltage andpower flow is controlled by the use of the large generators since they are all equipped withvoltage control facilities and power control. Further downstream the voltage is controlled bymeans of tap changers on the transformers.
Voltage control in a transmission system to a high degree is dependant on the production andconsumption of reactive power. In a simplified way one can describe the transmission systemas being mainly inductive and thus absorbing reactive power causing a voltage drop.Furthermore many loads are inductive, consuming reactive power, and thus further lowering
the voltage. Without a proper voltage control the electrical power system will not functionsince all loads are designed to receive voltage within a specific range. This means that at theconnecting point of large offshore wind farms with the main electrical grid the reactive powerhas to be balanced under normal operating conditions. In the past in the event of faults windfarms were disconnected immediately from the grid. Now, with wind farms growing into therange of big or medium sized power plants contribution to the short circuit fault current willbe required from new wind farms. To support the system and in order to reestablish normaloperation after faults in the power system as fast as possible the wind farms must remainconnected to the grid as long as possible.
Main configurations for connections of offshore wind farms to the gridTwo alternatives are offered for connecting offshore wind farms to the main grid: AC andDC.
A major advantage of AC is the low station cost. However, with growing distance the cablecost become significant and above a certain distance prohibitive. Long AC cables producelarge amounts of capacitive reactive power. The charging current of the cables reduces thetransmission capacity more and more. The capacitive power of the cable needs to be balancedby inductive reactive power in order not to create problems with high voltages. Parameters to
optimize an AC connection are the transmission voltage level and the number of cablesystems to be applied.
AC connection means synchronous operation of the wind farm and the grid. All faults in themain grid directly affect the collecting AC grid offshore and vice versa . To mitigate thisdynamic effect fast voltage control needs to be provided as is demonstrated below.
DC has the advantage of lower cost for cables and lower cable losses above a certain distanceand thus compensates the high converter costs. So for longer distances DC becomescompetitive for the investments as well as for the operating costs.
In addition the DC transmission generally decouples both grids, so that, for example, it allows
asynchronous operation of the offshore wind farm AC network and the main grid. Thisfacilitates, in case of faults in the network, a fast return to prefault power transmission. UsingVoltage Source Converters ( VSC ) additional features are the possibility of island operationand black start. So DC offers more flexibility to support the main power system.
The system approach on designing a connection for an offshore wind power farm identifiesamong different configurations the most suitable and economical one.
The AC solution
Stationary voltage control using VAr compensators for wind farms
A major issue of AC connections is how to provide adequate reactive power control. Forstationary operation mainly the symmetrical, relatively slow, reactive power control isimportant. The reactive power of the cables have to be compensated depending on the load.Maximum compensation is required at no load condition. During operation at rated poweronly about 50 to 60% of the no load compensation power is needed. The compensationequipment thus consists of two elements, one with fixed compensation and another withcontrolled compensation, both inductive. Fixed compensation requires less space thancontrolled compensation equipment and, therefore, is preferably placed offshore. Or in smallunmanned stations behind the coast line. To maintain the voltage offshore within prescribedlimits the voltage at the onshore end of the cable can be controlled. In addition the generatorsof the wind turbines can provide to some extent reactive power and voltage control.
Fig. 1 shows a typical configuration for an AC connection from an offshore wind farm to themain grid.
Figure 1. Principal configuration of an AC connection of a wind farm to the main grid
In the grid connection point a centralized approach using Static VAr Compensator (SVC)seems to be attractive. There the stationary production of reactive power provides thenecessary balance of reactive power and controls the voltage in the grid connection point,making it possible to transmit the desired power levels. It is preferred to use some kind of regulating scheme continuously controlling the reactive power injection in the power grid as afunction of the active power generated.
Dynamical issuesThe reliable operation of power transmission systems depends to a high degree on thebehavior of the connected loads and generators during and after faults have occurred in thegrid. These faults are mainly temporary such as single line to ground faults caused bylightning. The faults are cleared by the protection schemes removing the affected line fromthe system. A typical case for existing wind farms would be a sub-transmission systemfeeding a substation close to the shore with existing outgoing lines from this substationserving small villages and industrial plants. A temporary fault on one of the outgoing lineswill lower the system voltage. If the fault is close to the substation the voltage willimmediately drop to almost zero. The system protection will identify the fault and the linewill be disconnected and system voltage will try to recover. If recovery of the system voltageis successful or not depends to a certain degree on the loads and the dynamical support of reactive power, see figure 2 [1]. Typical loads being inductive and containing some inductionmotors will require large amounts of reactive power support to recover. When a newgenerating facility, such as a wind farm, is connected to the system it is extremely importantto make sure the wind farm is designed to support the system during the fault. Depending onthe type of wind turbine generators to be used, induction / double-fed / synchronous, thebehavior during and after faults will be different, but all types need to be investigatedcarefully. The use of a centrally placed SVC improves the situation for all turbine types.
Figure 2: Typical grid voltage behavior during and after a fault
Not only the almost instantaneously support of reactive power during and after fault, but also
the possibilities of rapid control of the reactive power make SVC and wind farm acombination which will increase the prospects of AC connected wind farms becoming a newworkhorse in the power generation business.
Beside the mentioned advantages the results of a real case study show what the systemapproach for an individual grid connection could mean:
Optimized design for combined connection of several wind farmsTwo wind farms, „ A „ and „ B „ are planned nearby in the Baltic Sea west of the islandRügen (Fig. 3). Several configurations of grid connection were studied in detail, beside otherit were the following:1. separate AC connections to shore2. combined AC connection to shoreAs a result of the system optimization studies the following was found :
Variant Main Component Connection „ A „ Connection „ B „1) separate con-nections
cables 2 cable systems 150 kV/ 200 MVAeach / 110 km sea, 10 km land
1 cable system 110 kV/60 MVA, 50 kmsea, 10 km land
reactive power
compensation
150 MVAr offshore,
150 MVAr onshore
30 MVAr onshore
2) combinedconnections
cables 2 cable systems 150 kV/200 MVAeach 110 km sea, 10 km land
included in connection of „ A „
reactive powercompensation
75 MVAr offshore at „ A „,150 MVAr onshore
+ 75 MVAr offshore at „ B „
Table 1. Two configurations for grid connections of the wind farms „ A „ and „ B „
Figure 4. Wind farms „ A „ and „ B „ and their grid connections
Table 1 summarizes the main technical data of the transmission cables for two possiblesolutions. Variant 2 with combined connection of the wind farms proves to be much moreeconomic since the combined power of the two wind farms can be transmitted to shore withthe same cable systems and the same total reactive power compensation as for the larger wind
150 kV
190 MVA
30 kV
30 kV
30 kV
30 kV
190 MVA
390 MVA
150 kV 400 kV
75 MVAr
Windpark „A“375 MW
150 MVArSVC
60+50 kmsubmar. cable
10 km
landcable
52 MW
60 km
submar.cable
50 km
10 km
landcable85MVA
75MVAr
SVC
„ A „93 WT
„ B „21 WT
Bentwisch Vattenfall Europe
375 MW
52 MW
400 / 110 kV
Rostock
~ 60 km
~ 50 km
150 kV
150 kV
BALTIC SEA
RÜGEN
Windpark „B“
Figure 3. „ SLD optimized grid connection for Wind farms „ A „ and „ B
farm of both alone. The at the first glance surprising reason is the better use of the AC cableswhere the reactive power compensation can be applied more evenly along the length of thetransmission, it can be interpreted as an addition of two shorter transmissions withcompensation on both ends.
The DC solutionWith wind power parks becoming a considerable share of the total power generation in anetwork, wind power will have to be as robust as conventional power plants and stay online atvarious contingencies in the AC network. As already shown, compensation will then beneeded to preserve power quality and/or even the stability in the network.
HVDC Light is a DC transmission system based on voltage source converter (VSC)technology which has characteristics suitable for connecting large amounts of wind power tonetworks, even at weak points in a network and without having to improve the short-circuitpower ratio [4].
HVDC Light does not require any additional compensation, as this is inherent in the controlof the converters (Fig. 5). From operation of installed systems experiences has been gainedwith HVDC Light transmission systems showing it as an excellent tool for bringing powerfrom windmill parks to a network and at the same time contribute to AC voltage stabilization.An overview of a transmission system for bringing offshore wind power to a network withHVDC Light is shown in figure 6. An HVDC Light transmission system can control theactive power transmission in an exact way, so that contracted power can be delivered whenrequested and when (of course) available from the wind situation. The power transmission canbe combined with a frequency controller that varies the power to override or support the net-work frequency controller.
Figure 5. Active and reactive power control
Figure 6. HVDC Light transmission system for offshore wind power
An HVDC Light converter controls reactive power for its AC bus and, in conjunction with amaster controller, AC voltage control of the network connected to the converter station can beprovided. Such AC voltage control can also be used to improve the power quality by includ-ing control of flicker and other transient disturbances.
In the case of connection to a passive network as for example a wind farm , the HVDC Lighttransmission system can provide control functions for active and reactive power, so that bothvoltage and frequency can be controlled from the converter station. In particular, this allowsblack starting by controlling the voltage and frequency from zero to nominal. HVDC Lightconnected to a wind farm could also give the possibility to provide reactive power to thewindmills during the start up.
An HVDC Light transmission has only one significant value connecting both grids with eachother: the real power in size and direction which is equivalent to the product U x I on the DClink. All the other physical values, which are typical for an AC grid (reactive power, apparent
power, frequency, harmonics, DIPS, SAGS and flicker, etc.) are decoupled and do not affectthe other grid.
Many of these values can be controlled and mitigated by intelligent control schemes from theconverter feeding the respective grid.
Experience with HVDC Light installationsTo date, six HVDC Light transmission systems have been contracted and put into operation.Another one is under construction to bring power from shore to the Troll A platform in theNorwegian part of the North Sea. These HVDC Light transmission systems are brieflypresented in Table 2 below. They represent a variety of applications such as interconnection
for trading, underground transmission for easy connection and transmission of wind power,power supply to platforms etc. In their operation they all take advantage of the comprehensivepossibilities for control that a VSC converter offers.
Three of them have features that will be of special interest for power transmission from off-shore wind power generation plants and connection to a network. Gotland and Tjaereborgboth connect wind power to a network and the Troll A has a converter designed for deliveringpower from shore to a platform in the sea.
Table 2. Existing HVDC Light transmission systems worldwide
The Gotland HVDC Light system rating is 50 MW and 65 MVA and it is connected inparallel with the existing 70 kV / 30 kV AC grid. The Gotland island system has a
peak load of about 160 MW and today there are a total of 165 windmills with a totalinstalled power of 90 MW producing about 200 GWh. The short circuit power fromthe AC grid is less than 60 MVA at the connection point in Näs, where the windpower production is connected for the HVDC Light system. The grid operator,Gotlands Energi AB (GEAB) considered, that HVDC Light would be the onlyrealistic way to solve the technical problems with the high amount of wind powerin-feed The experiences have supported expected improvements in characteristicssuch as:
- Flicker problems were eliminated with the installation of HVDC Light andtransient phenomena disappeared.
- Stability in the system arose.- Power flows, reactive power demands, as well as voltage levels in the system
and harmonics were reduced.One result is that the voltage stability during transient events has become much betterwith HVDC Light, which improves the output current stability from theasynchronous generators. This reduces not only the stresses on the AC grid but alsoon the mechanical construction of the windmills. Overall experiences are that thecontrol of power flow from the converters makes the AC grid easier to supervise thana conventional AC network and the power variations do not stress the AC grid asmuch as in normal networks. Voltage quality has also been better with the increasedwind power production [3].
TheTjæreborg wind farm consists of four wind turbines (WTs) with a total installed capacityof 6.5 MW and is a test installation. The DC pole cables have been installed inparallel with the existing AC cables, the sending end converter is installed at thewind farm, and the receiving end converter is installed in the Tjæreborg substation.The purpose was to investigate how the controllability of the VSC transmission aswell as optimal exploitation of the wind energy by using the converter for providinga collective variable frequency to the WTs.Simulation of three-phase faults demonstrated that the DC connection has the
potential to improve wind farm performance during faults in the AC grid. The windfarm can be quickly isolated from the AC grid and rapidly recovers to full windpower production when the AC grid fault has been restored. It implies that theconverters at both sides of the DC-transmission can stay in operation and connectedto the grid, when a fault in the other grid occurs. Thus the requirement of shortestpossible interruption in case of faults can optimally be obtained. Testing has shownthat the converter station smoothly varies the frequency and that the frequency at thesending end can be controlled solely by the converter station, while the frequency inthe receiving end is the AC grid frequency [4].
On theTroll A platform, two HVDC Light transmission system for 45 MW, +/-60 kVdirectly feed two high-voltage variable-speed synchronous machines designed forcompressor drive with variable frequency and variable voltage with power from land[5]. On the platform equipment will be installed in housings that will be lifted on tothe platform. Space and weight have to be kept to a minimum on an offshore
installation. The HVDC Light concept therefore offers important advantages andthanks to smaller filters than conventional HVDC and no need for additional reactivepower generation equipment it can be made compact and lightweight. The layout iskept compact on the platform by placing the converter equipment in a multi-storeymodule. The HVDC Light offshore converter is planned to be built as a prefabricated
unit and transported and installed on top of the platform. The structure will haveapproximate main dimensions of W x L x H = 18 x 17 x 14 meters.
Figure 7. Multi-storey 350 MW converter designed for platform applications
For large wind farms an HVDC Light system of maximum rating, currently 350 MW, willprobably be of more interest than the Troll A 45 MW units. A tentative design for such aconverter in a three-storey enclosure has been made and would measure 30 x 40 x 20 metres.This would also include the transformer, which would be needed for connection to thewindmills.
HVDC Light is a transmission system with direct current and thus has no technical limitationswith regard to distance. This makes it the natural choice for long transmissions, when thedistance becomes too long or uneconomic for AC transmission.
The operational characteristics of HVDC Light, which have been experienced in theoperation, together with wind power as related above, makes it a strong alternative in manyother situations, such as:
• infeed of wind power in points in the network, which are weak and/or causes problemswith flicker, stability or other quality issues
• operation of the wind mill park at different and/or varying frequency, e.g. for optimum use
Vast experiences of both AC and DC transmissions show that both technologies are availableand feasible for connection of large offshore wind power plants to a grid. A supplier withexperiences from both technologies and having all components would be able to assist in thechoice of transmission and connection system. All mentioned realized examples are projectsresulting from feasibility and detailed studies of discussions, calculations and bench marks tofind system optima in a long and trustful cooperation with customers and internal and externalsub suppliers.
The results prove the quality of the system optimization process.
REFERENCES
1. Noroozian, M., Knudsen, H., Bruntt, M.: Improving a wind farm performance by reactivepower compensation. Proceedings of IEEE Summer Meeting 1999, Singapore
2. Eriksson K: Operational experience of HVDC Light; IEE Seventh InternationalConference on AC-DC Power Transmission; London, November 2001.
3. Axelsson U, Holm A, Liljegren C, Åberg M, Eriksson K, Tollerz O: The Gotland HVDCLight project – experiences from trial and commercial operation; CIRED 2001,Amsterdam, Netherlands, June 2001.
4. Soebrink K, Soerensen A, Jensen J, Holmberg P, Eriksson K, Skytt A-K: Large-scaleoffshore wind power integration and HVDC using Voltage Sourced Converter; CigreFourth Southern Africa regional conference, Cape Town, South Africa, October 2001.
5. Hörle N, Eriksson K, Maeland A, Nestli T: Electrical supply for offshore installationsmade possible by use of VSC technology; Cigre 2002 Conference, Paris, August 2002.
6. Ana Díez Castro, Rickard Ellström, Ying Jiang Häffner, Christer Liljegren. Co-Ordinationof Parallel AC-DC Systems for Optimum Performance. Power Delivery Conference,September 1999, Madrid.
